Superconductivity and electronic state of annealed single-crystals of FeSe_1-xTe_x (0.6 ≦x≦1) studied by specific heat

We have investigated the superconductivity and electronic state in FeSe_1-xTe_x (0.6 ≦ x ≦ 1) from the specific heat measurements, using annealed single-crystals of good quality. First, it has been found that annealed single-crystals of x = 0.6-0.9 exhibit bulk superconductivity with a clear specific-heat jump at the superconducting (SC) transition temperature, T_c. Both 2Δ₀/k _BT_c [Δ₀: the SC gap at 0 K] and Δ C/(γ_n-γ₀)T_c [Δ C: the specific-heat jump at T_c, γ_n: the electronic specific-heat coefficient in the normal state, γ₀: the residual electronic specific-heat coefficient at 0 K in the SC state] are largest at x = 0.7 and are 4.29 and 2.38, respectively, indicating that the superconductivity is strong-coupling. Secondly, it has been found that the γ_n value is much larger than that estimated from the band calculation and increases with increasing x and suddenly decreases above x = 0.9 due to the antiferromagnetic ordering around x = 1. The large value of γ_n is guessed to be due to the enhancement of the effective mass related to spin fluctuation and/or orbital fluctuation. Finally, it has been found that there remains a finite value of the electronic specific-heat coefficient at 0 K even in the SC state at x = 0.8-0.9. This suggests that a phase separation into SC and normal-state regions takes place at x = 0.8-0.9. Otherwise, there might remain a normal-state pocket at the Fermi surface even in the SC state.